An Orthogonal Frequency Division Multiplexing (OFDM) scheme is a digital modulation scheme that is efficient in wideband digital communication systems. The OFDM scheme is applied in digital transmission/reception systems, such as European digital audio broadcasting (DAB), digital video broadcasting (DVB), asymmetric digital subscriber line (ADSL), wireless local access network (WLAN), and Wibro.
The OFDM scheme is robust against intersymbol interference (ISI), which is an important problem in high speed communication. In addition, the OFDM scheme converts frequency selective fading into frequency nonselective fading.
However, compared with a single carrier system, the OFDM scheme is sensitive to an oscillator mismatching between a transmitter and a receiver or a carrier frequency offset caused by a Doppler frequency shift.
In the OFDM system, the carrier frequency offset destroys the orthogonality between subcarriers. Because even a small carrier frequency offset causes a significant degradation of a system performance, a frequency synchronization is one of the most important issues in a system configuration.
In addition, because “eye opening” does not exist which can find an optimal sampling time, the OFDM system must use a different time/frequency synchronization scheme from a symbol synchronization used in a single carrier system. In the OFDM scheme, a time synchronization means finding an initial estimation value of a single OFDM symbol. Because the OFDM uses a cyclic prefix (CP), it is less sensitive to a symbol synchronization error, but must accurately estimate a frequency offset so that it cannot exceed an allowable error range of the CP.
A conventional time synchronization method and frequency offset estimation method used in the OFDM system can be classified into a blind type method and a data, pilot or CP dependent method.
A pilot dependent time/frequency synchronization method is disclosed in U.S. Pat. No. 5,732,113 entitled “Timing and frequency of OFDM signals, a CP dependent time/frequency synchronization method is disclosed in a paper entitled “ML Estimation of Time and Frequency Offset in OFDM Systems”, IEEE Transaction on Signal Processing, Vol 45, No. 7, pp. 1800-1805, and a blind time/frequency synchronization method is disclosed in a paper entitled “OFDM blind carrier offset estimation”, IEEE Transaction on Communications, Vol. 48, No. 9, pp. 1459-1461.
The pilot dependent time/frequency synchronization method uses two pilot symbols in order to both an integer part and a decimal part of a frequency offset. However, because the two pilot symbols must be inserted into a transmit signal, a data rate is reduced and there exists an ambiguity of a time synchronization estimation corresponding to a CP length in the time synchronization.
The CP dependent time/frequency synchronization method uses a correlation between a data symbol part and a CP part. This method cannot estimate an integer part of the frequency offset and cannot detect a start point of a frame, although a symbol synchronization may be achieved.
The blind time/frequency synchronization method uses a virtual carrier instead of a pilot symbol or CP. This method increases the complexity due to a vector space calculation.
In a satellite communication network, receive/transmit channels experience a rician fading having a Line of Sight (LoS) component. Thus, the conventional time/frequency synchronization method considering a terrestrial mobile communication environment, e.g., a Rayleigh fading channel, is inefficient in the satellite communication network.
Therefore, there is an increasing demand for a time/frequency synchronization method adapted for an OFDM-based satellite communication system. Specifically, a structure of a preamble transmitted from a transmitter to a receiver for a time synchronization and a frequency offset estimation must be designed efficiently. In addition, there is a demand for a method that can acquire a time synchronization using the preamble and correctly estimate the frequency offset based on the time synchronization.